This invention relates to apparatus and a method for controlling torque applied to a fastener to achieve a predetermined final tension load on the fastener, or on each fastener of a group of like fasteners being simultaneously operated upon, and to constantly monitor torque and efficiency on each fastener of the group to assure that final tension load is maintained thereon until final tension load is developed on all fasteners of the group.
New and improved methods, or systems, are constantly being developed and utilized in the fastener tightening art. Most methods currently employed, operate to control torque to achieve desired ultimate fastener tension. In assembly operations, involving multiple fasteners, torque control monitoring permits the assembly to be checked to assure that all fasteners being operated upon attain predetermined final tension value before torque loading is terminated. However, the value of tension on a given fastener in an assembly operation depends upon certain variables such as friction and load in the assembly.
For any given fastener there is a constant relationship between fastener elongation, fastener tension or load, and fastener rotation relative to its fixed threaded counterpart, which relationship is not affected by friction. Some of the known fastener tightening systems attempt to utilize this relationship to control fastener tension. However, the means used to apply this relationship are affected by friction. Examples of frictional effects in known tightening systems are:
1. The known systems assume that the torque-angle curve is a straight-line, for the cycle portion where all rotation is absorbed by elongating the fastener. This straightline is used to establish a fastener's zero load point, from which the tightening angle is measured. If the frictional effects are not constant the straight-line assumption is incorrect and the fastener will not be tightened correctly.
2. Some of the known systems use the yield point of the torque-angle tightening curve to correlate tightening angle with bolt tension. However, yield point of a fastener, as developed on an actual torque-angle curve, is a result of stresses produced from fastener tension load as well as friction.
Another deficiency of the systems which control fastener rotation to achieve final tension, is that there is no way to check a completed assembly to determine if it has been tightened to the correct angle of rotation to obtain a tension specification.
The subject invention incorporates development of a theoretical torque-angle curve which is used to establish calculation of an efficiency factor. Such a curve is not used in known systems of the prior art. The converting of actual torque to fastener tension through such an efficiency factor and a fastener constant, is more accurate than relating an actual fastener yield point to bolt tension. Locating a theoretical curve with respect to the actual torque angle curve is simpler and more accurate than projecting the shape of the actual curve back to final zero load angle and then rotating to a predesignated angle to achieve desired final torque.
Development and use of the efficiency as disclosed herein allows measurement of fastener torque with a torque wrench the value of which, when multiplied by efficiency and a fastener constant, will determine the clamp load of the fastener under consideration. Such will allow a check of the system itself to determine if desired fastener tension has been developed, or can be later used to tighten a fastener to the torque required to achieve desired level of clamp load.